This invention relates generally to nuclear reactors and, more specifically, to a means to prevent crud buildup at the interface of the end plug and tie plate in a fuel bundle for a nuclear reactor.
The release of large amounts of energy through nuclear fission reactions is now quite well known. Nuclear reactors are presently being designed, constructed, and operated in which the nuclear fuel is contained in the fuel elements which may have various shapes, such as plates or rods. For convenience these fuel elements will hereinafter be referred to as fuel rods. These fuel rods are usually provided on their external surfaces with a corrosion-resistant nonreactive cladding. The fuel rods are grouped together at fixed distances from each other in a coolant flow channel or region as a fuel bundle, and a sufficient number of these fuel bundles are combined to form a nuclear reactor core capable of the self-sustained fission reaction. The core is ordinarily contained within a reactor vessel. Such nuclear reactors are now widely known and are discussed in greater detail by, for example, M. M. El-Wakil, "Nuclear Power Engineering", McGraw-Hill Book Company, Inc., 1962.
In general, nuclear power plants are designed for periodic shutdown to refuel the reactor core. This is referred to as "reloading" the reactor and is performed by replacing part or all of the irradiated fuel with unused reload fuel. Typically, the reload schedule is arranged for reactor shutdown during those periods when power demands on the overall power grid are at a minimum. The scheduled reload may typically require that 20 to 25 percent of the irradiated fuel be removed from the reactor core and replaced with reload fuel. Therefore, for 25 percent annual reloads, there will be four reload shutdown cycles resulting in a complete replacement of the original fuel at the end of four cycles of normal operation.
The nuclear design of the reload fuel is fixed many months (12 months is not unusual) before loading the reload fuel into the reactor. The major portion of this lead time is required for nuclear design, manufacturing, licensing and delivery of the reload fuel. It is very important to note that the nuclear design of the reload fuel is based on the condition of the reactor that is predicted to exist at the date of the scheduled reload. The basic conditions which must be considered in designing the reload fuel are: (1) the reactivity condition of the reactor core and remaining fuel, (2) the design lifetime and reactivity of the reload fuel, (3) the control rod strength (the neutron absorption effectiveness of the control rod), and (4) the desired shutdown margin (the control rod system strength over and above that required to shut down the reactor).
Since the reload fuel is designed many months in advance, there is a substantial likelihood that the nuclear design will not strictly meet the needs of the reactor at the time of reactor shutdown if unexpected deviations from originally predicted conditions occur. Such deviations may result, for example, from operating at power levels above or below that assumed for the period. From an economic operation standpoint, it would be very desirable to change the characteristics of the reload fuel to accommodate these deviations in order that the reload fuel meet the needs of the reactor as closely as possible. Once the reload fuel has been manufactured to a design correct for the assumed reactor conditions, there have been available prior to the present invention, only very limited and laborious techniques for changing the nuclear characteristics of reload fuel to accommodate the deviations from the assumed conditions so as to meet the needs of the reactor actually encountered at the time of shutdown for refueling. In such situations the time and expense required to alter the reload fuel bundles may be such as to require the use of the unmodified reload fuel under inefficient conditions. It may also require the unplanned exchange of control rods to increase or decrease their reactivity worth, or it may require the rearranging of fuel bundles in the core, or both. These situations can often require excessive expenditures of money, extended shutdown of the reactor power plant or other inconveniences which are detrimental to the economic and efficient operation of a nuclear reactor power plant.
In addition, it is sometimes necessary to remove the irradiated fuel bundles from the reactor prior to the completion of their scheduled exposure period. This unscheduled removal may be due to such factors as mechanical failure of one or more of the fuel rods in the fuel bundles or unexpected changes in the physics characteristics or requirements of the fuel bundle or reactor core. It had been general practice in these situations to scrap the fuel bundle since it was radioactive and could not be readily modified or repaired. In U.S. Pat. No. 3,431,170, issued on Mar. 4, 1969, to James L. Lass and Dominic A. Venier, and assigned to the assignee of this invention, there is disclosed an improved fuel bundle which may be readily repaired or modified while the fuel bundle is in the reactor core or when it is in a separate containment such as a water pool adjacent the reactor vessel. That fuel bundle included an easily removable upper tie plate and a plurality of easily removable fuel rods which may be removed or inserted after removal of the upper tie plate. This was achieved by employing specially designed upper and lower fuel rod end plugs and a removable locking mechanism for the upper tie plate. The lower end plugs were shaped to permit insertion and withdrawal of the fuel rod without binding or catching on the fuel bundle spacers. This was achieved by the unique cooperation between a self-centering cylindrical surface area and specially formed conical surfaces. The upper end plugs were shaped to receive detachable tooling which could be attached thereto to raise and lower the fuel rods.
Operating experience has shown a tendency for the buildup of contaminants (referred to as "crud" by those skilled in the art) in the area where the end plugs mate with the tie plates. The crud tends to bind the end plug and tie plate together thereby hampering their separation and removal of the fuel rod from the bundle.
Accordingly, it is an object of this invention to provide a means to minimize crud buildup during reactor operation at the interface between the end plug and the tie plate.